The proposed studies will examine the influence of altered activation of the diaphragm of the fatigue resistance and oxidative capacity of muscle units. Prolonged diaphragm disuse will be imposed by blocking neural traffic in the right phrenic nerve using tetrodoxtoxin (TTX). In both control and TTX treated animals, diaphragm motor units will be isolated by microdissection of C5 ventral root filaments. Diaphragm motor units will be classified into different types, based on standard criteria commonly used in the study of hindlimb motor units, e.g. fast- and slow-twitch motor units will be distinguished using the "sag" test and a 2-min fatigue test will be used to separate units into fatigable, intermediate and fatigue resistant types. Muscle fibers will be classified as Type I, IIA or IIB based on differences in staining for myosin ATPase at different preincubation pH's. The oxidation capacity of muscle fibers will be determined by quantifying the histochemical reaction for succinate dehydrogenase (SDH) using a microphotometeric technique. The cross-sectional areas of muscle fibers will also be quantified. In population studies, the influence of prolonged disuse of the diaphragm on the distribution of: 1) Motor unit types; 2) Muscle fiber types; 3) Fiber SDH activities; and 4) Fiber cross-sectional areas will be determined. In another series of studies, muscle fibers belonging to single motor units in both control and TTX treated diaphragms will be identified using the method of gylcogen depletion. The variability of SDH activities among muscle unit fibers will be compared to that for non-unit fibers. The relationship between motor unit fatigue resistance and the mean SDH activity of muscle unit fibers will be determined. In these studies, we will control for the potential influence of precontractile failure of some muscle fibers by comparing the properties of motor units that show a decrease in motor unit action potential (MUAP) amplitude during the fatigue test with those of units showing no change in MUAP amplitude. The results of these studies will be clinically important for several reasons. First, the long-term maintenance of patients on mechanical ventilators may seriously impair the contractile and fatigue properties of the diaphragm. Secondly, detremental adaptive responses of the diaphragm to altered levels of activation may contribute to the pathophysiology of certain respiratory diseases. Finally, understanding how the diaphragm adapts to altered levels of activation may provide the basis for designing treatments effective in improving ventilatory muscle strength and endurance.